Staff: Admin

How would you respond to a student who raises the issue of "frame of reference" for which the definition of "axis of rotation" is made?

For example if a student said in the context of a coin rolling on a table at a constant velocity, "What about from the frame of reference of the center of gravity of the coin? The table is moving in a straight line relative to the COG, the edge is moving without slipping at the same speed as the table, but the coin is rotating about the COG which is stationary (translationally). Why in this frame of reference should the COG not be considered the axis of rotation?"

How would you respond to a student who raises the issue of "frame of reference" for which the definition of "axis of rotation" is made?

For example if a student said in the context of a coin rolling on a table at a constant velocity, "What about from the frame of reference of the center of gravity of the coin? The table is moving in a straight line relative to the COG, the edge is moving without slipping at the same speed as the table, but the coin is rotating about the COG which is stationary (translationally). Why in this frame of reference should the COG not be considered the axis of rotation?"

How would/should you respond?

Indeed, in the frame moving with the coin, the COG is the axis of rotation. The fact that in the moving frame of reference it is and in the stationary frame it is not gives rise to the preconception that I attempt to explain in the first paragraph of "Whence the preconception?" Let me rephrase my hypothesis. In the moving frame of a rolling wheel, the axis of rotation is the center of the wheel. An observer in this moving frame "knows" that (s)he and not the ground is moving. Put the same observer at rest on the ground. When (s)he sees a wheel roll by, (s)he carries over the mental image of the axis of rotation being at the COG and doesn't even consider that this axis of rotation in the stationary frame is never at rest, not even instantaneously. On one hand most people are unaware of the physics definition of the axis of rotation. On the other hand, they find it more convenient to think of the COG as always being the axis of rotation. They are intuitively familiar with axes of rotation that are stationary relative to them (spinning turntable, opening door, etc.), but have a difficult time visualizing an instantaneous axis of rotation. You and I and everybody else are perfectly content driving our cars and riding our bicycles without giving a hoot about where the axis of rotation is. Nevertheless, students in a physics class ought to know the correct way of looking at rolling motion.

As referred to in AT's post, in the insights "verification" section, one reason for which way the cable spool moves in response to where a force applied to the side arm has to do with effective gearing. The side arm could be replaced with an inner cylinder 1/2 the radius of the cable spool, and geared to rotate at 3 to 4 times the rate of the spool. In this case a horizontal tension applied to a string wrapped under and around the inner cylinder, would result in the spool moving in the opposite direction of the string, even though the force is being applied at 1/2 the radius above the ground that the spool rolls on. This is the same reason that the second walkway cart in AT's second video moves in the opposite direction. Not mentioned in that video is the carts speed relative to the walkway is related to the gear ratios. Say the video gear ratios are 2:1 or 1:2. With gear ratios of 1.5:1 or 1:1.5, the carts would move faster, and with gear ratios of 3:1 or 1:3, the carts would move slower.

Another issue to consider is the centripetal acceleration experienced by the particles of the cable spool, and where is the axis related to that centripetal acceleration. You could consider a rotating wheel in space moving at constant velocity (no linear acceleration) free from any external forces. The internal tensions within the rotating wheel related to centripetal acceleration are independent of the frame of reference.

Indeed, in the frame moving with the coin, the COG is the axis of rotation. The fact that in the moving frame of reference it is and in the stationary frame it is not gives rise to the preconception that I attempt to explain in the first paragraph of "Whence the preconception?" Let me rephrase my hypothesis. In the moving frame of a rolling wheel, the axis of rotation is the center of the wheel. An observer in this moving frame "knows" that (s)he and not the ground is moving. Put the same observer at rest on the ground. When (s)he sees a wheel roll by, (s)he carries over the mental image of the axis of rotation being at the COG and doesn't even consider that this axis of rotation in the stationary frame is never at rest, not even instantaneously. On one hand most people are unaware of the physics definition of the axis of rotation. On the other hand, they find it more convenient to think of the COG as always being the axis of rotation. They are intuitively familiar with axes of rotation that are stationary relative to them (spinning turntable, opening door, etc.), but have a difficult time visualizing an instantaneous axis of rotation. You and I and everybody else are perfectly content driving our cars and riding our bicycles without giving a hoot about where the axis of rotation is. Nevertheless, students in a physics class ought to know the correct way of looking at rolling motion.

I agree but would add that different problems can have different correct ways of looking at them. Øyvind Grøn's paper 'Space geometry in rotating reference frames: A historical appraisal', ( http://areeweb.polito.it/ricerca/relgrav/solciclos/gron_d.pdf ) is a useful compendium of various different approaches.

Grøn also gives a calculated solution to one relativistically rolling wheel problem in the paper. The part C 'optical appearance' plot, at retarded points in time, provides

The positions of points on a rolling ring at retarded points of time were calculated with reference to K0 by Ø. Grøn [111]. The result is shown in Fig. 9. Part C of the figure shows the “optical appearance” of a rolling ring, i.e. the positions of emission events where the emitted light from all the points arrives at a fixed point of time at the point of contact of the ring with the ground. In other words it is the position of the points when they emitted light that arrives at a camera on the ground just as the ring passes the camera.

I have only seen one complete verification of Grøn's part C plot (on another forum) and the process is interesting considering the basic parameters; no z axis (no Born rigidity issues), x, y (all in t) only with wheel, axle (or carriage) and road frames at any specific time. The specific time gives us the location of the axle anywhere over one complete rotation and the velocity of the wheel gives us the length contracted location of the emission point wrt the axle and its location and also the fixed observer (camera) location on the road ahead. The photons just have to travel straight from their emission point to the camera at c.

One major elements missing from the part C figure are the straight line photon paths from each emission point to the camera. The length of these lines can represent the actual time that each photon travels for and, for each emission point, this time must also equal the time that the wheel will take to roll from its emission point axle location to the camera point along the road. As a final cross check you can compare the axle locations and times between consecutive emission points to see if they match the angular velocity of the emission point/spoke tip in part and in all.

Note that while the COG is used in the calculations its use here is as a hypothetical central axle for hypothetical relativistically rolling wheel spokes that had emission points at their tips. Grøn describes the emission points as being on a rolling ring, not a wheel with spokes, although the plot results appear the same.

This issue may be a little more subtle than your explanation so far. Since you don't explicitly say anything about the reference frame when you ask the original question, an answer framed from the point of view of someone on the bicycle is just as correct as an answer framed from the point of view of someone on the ground.

The preconception you mention, is not necessarily a misconception, it is simply a perception from a different point of view, a different reference frame.

> one “knows” who is “really” moving and it is not the ground
It almost sounds like you are thinking that the person on the surface of the earth is in a fixed, preferred, reference system. In reality, all we know about the reference frame of the bicycle and the reference frame on the surface of the earth is that they are in motion relative to one another. Neither is truly stationary, and neither is preferred.

Neither of these reference frames is preferred, and the laws of physics are the same as measured from either frame. These are incredibly important concepts in physics and this example could be a powerful and memorable example for students. So rather than talking about which axis of rotation is correct or incorrect, include how it varies with reference frame.

An observer in this moving frame "knows" that (s)he and not the ground is moving.

To follow up on Alex Kluge post about frame of reference, take the case where a coin is rolling on a treadmill such that the center of mass of the coin is not moving with respect to ground, but instead the treadmill is moving with respect to the ground.

> spool trick

If the spool was on rails and the hub had a larger radius than the sides of the spool, then pulling the string from under and back from the spool would result in the spool moving forwards. The hub could also be geared so that it rotated faster than the spool, so that string speed is greater than spool rolling speed, and again the spool would move forwards. This would be the same as the second walkway cart at 1:28 into the second video of AT's post.

> point of rotation from ground based frame of refernce

To follow up on my prior post, from a ground frame of reference, a point on a wheel follows the path of a cycloid, where the instantaneous acceleration at the point of contact would be zero, and It might be mathematically convenient to use the point of contact as the pivot point, but this conflicts with the tension within the wheel, which corresponds to the centripetal acceleration of all points about the center of mass, regardless of which inertial frame of reference is being used. In my opinion, both views are mathematically correct for a ground based frame of reference, rotation about the contact point, or a combination of linear motion at the center of mass and rotational motion about the center of mass.

This issue may be a little more subtle than your explanation so far. Since you don't explicitly say anything about the reference frame when you ask the original question, an answer framed from the point of view of someone on the bicycle is just as correct as an answer framed from the point of view of someone on the ground.

The preconception you mention, is not necessarily a misconception, it is simply a perception from a different point of view, a different reference frame.

> one “knows” who is “really” moving and it is not the ground
It almost sounds like you are thinking that the person on the surface of the earth is in a fixed, preferred, reference system. In reality, all we know about the reference frame of the bicycle and the reference frame on the surface of the earth is that they are in motion relative to one another. Neither is truly stationary, and neither is preferred.

Neither of these reference frames is preferred, and the laws of physics are the same as measured from either frame. These are incredibly important concepts in physics and this example could be a powerful and memorable example for students. So rather than talking about which axis of rotation is correct or incorrect, include how it varies with reference frame.

When I show this series of demonstrations, the default reference frame is the audience's frame which is fixed to the surface of the Earth. I did not mean to convey that I am thinking "that the person on the surface of the earth is in a fixed, preferred, reference system." I meant to convey that most people have an intuitive bias favoring the Earth's frame regardless of whether they are moving relative to it or not. Physics teaches us that there is no preferred frame. We understand this lesson intellectually but not intutively. For example, say I board a train in New York City headed for Boston. While the train is moving, I dismiss the train's frame of reference, with respect to which I am at rest, and I think of myself as going to Boston, not Boston coming to me. Furthermore, it is a good bet that my fellow passengers share this ambivalent view of reference frames and, if asked, they too will say you that they are going to Boston. The assumption that the Earth is at rest, while we go about our business moving on its surface, is the human way of viewing the world and has become a de facto preferred frame. We have known for centuries that the Earth spins about its axis, yet we still say that the Sun rises in the East and nobody bats an eyelash. Although physics does not have preferred reference frames, everyday life does.

Lest this reply be misunderstood, let me reiterate that I do not advocate preferred frames. On the contrary, I do other demonstrations designed to challenge students to see things from other frames of reference including non-inertial ones. These might be the subject of another "Insight" entry.

Neither of these reference frames is preferred, and the laws of physics are the same as measured from either frame. These are incredibly important concepts in physics and this example could be a powerful and memorable example for students. So rather than talking about which axis of rotation is correct or incorrect, include how it varies with reference frame.

The symetry of reference frames becomes clear when you consider the walkway carts below, from the rest frame of the walkway. The carts then basically swap their functionality: The one that moved in the same direction (but faster) as the surface moving in the chosen frame, now moves opposite to it. And vice versa.

You could show the same even simpler with a spool on a paper strip, or gears on racks, and consider the rest frame of the strip / upper rack:

For example, say I board a train in New York City headed for Boston. While the train is moving, I dismiss the train's frame of reference, with respect to which I am at rest, and I think of myself as going to Boston, not Boston coming to me. Furthermore, it is a good bet that my fellow passengers share this ambivalent view of reference frames and, if asked, they too will say you that they are going to Boston. The assumption that the Earth is at rest, while we go about our business moving on its surface, is the human way of viewing the world and has become a de facto preferred frame.

It's more subtle than that, I think. Watch someone pouring a drink on the train. They use an inertial approximation to the train frame for that - no-one is carefully considering the effects of trying to pour a drink at high speed in the Earth frame, but when the drink spills they will curse the rocking of the train and not the sudden appearance of fictitious forces. Although I bet if you ask they'll tell you that the table is stationary and not rocking, for which they use the true (non-inertial) train frame. Listen to people looking out of the window and you'll hear both the Earth-centric "we're going really fast" and the train-centric "the houses are going past really fast".

As you say, people adopt different reference frames without thinking about it. Disciplining yourself to pick one and stick with it, or at least to only switch consciously, is a big part of any non-trivial dynamics problem, in my opinion.